A standard information bus used in current computer systems for connecting computers and peripheral devices is the Small Computer System Interface ("SCSI"). A single SCSI bus can generally support up to seven peripheral devices on an 8 bit bus, and up to fifteen on an 16 bit bus, in addition to the host. The host is typically a personal computer or workstation, and the peripheral devices can include equipment such as auxiliary information storage devices (e.g. a hard disk drive or tape storage devices), CD-ROM drivers, and the like, as well as other computers. SCSI can be implemented using both single-ended or differential drivers and receivers, and in both a standard (8 bit) and a wide (16 bit) formats. SCSI bus protocols are set out in detail in ANSI specification X3.131-1986 for SCSI I; X3-131-1994 SCSI II, and in this application "SCSI I" or "SCSI II" refers to buses meeting at least the foregoing respective specifications (whether for single-ended or differential configurations), respectively, unless a contrary intention appears. Those references and all other references cited herein are incorporated in this application by reference.
The SCSI bus includes a set of conductive lines which carry the information signals. Signals that travel on the bus between devices have wave-like properties. As a result, signals that come to an end of the bus can be reflected back along the bus and interfere with other transmitted signals and possibly corrupt information carried by the bus. To maintain characteristic impedance to reduce reflections on the bus, it is well known to terminate each end of the bus using either active or passive termination. For this purpose, the SCSI bus also carries a conductive line (referenced as "TERMPWR") to convey termination power between terminators on different devices. Each information carrying line or control signal line is generally terminated using a suitable resistor configuration in a manner such as shown in FIG. 1 for a single-ended configuration, or in a manner such as shown in FIG. 2 for a differential configuration, or by using active equivalents. For an SCSI bus, such terminator configurations generally require about 5 volts at 1 amp to operate. Again, this power is provided typically from the host device through a TERMPWR line.
An SCSI bus has a maximum recommended length of about 6 meters for a single ended configuration, and up to 25 meters for a high-powered differential configuration. Beyond the maximum recommended lengths signal degradation becomes significant with increased possibility of errors. Of course, bus repeater configurations supported by motherboards, such as that described in U.S. Pat. No. 4,604,689, can be used to extend the distance over which a signal can be transmitted. However, such configurations are inconvenient in that they require relatively complex intermediate devices supported by on-board power supplies with connections to external power sources (typically 110 or 220 volt power lines).
Furthermore, as is apparent from above, the differential configuration is preferred in those situations where long bus lengths are required (for example, devices are to be connected by long bus cables). Often though, a device is provided with a bus of the single-ended configuration or a number of devices are already connected on a single-ended bus, and it is desired to connect another device at a large distance from the single-ended bus. In this case a bus converter must be provided. However, power must then be provided for the bus converter. The amount of power is not insignificant particularly where the converter must provide a differential bus operating with high powered transceivers. The conventional approach is to provide a converter device supported with a connection to a power outlet, such as 110/220 volt power connection. In complex SCSI network configurations with many devices, where one or more bus repeaters and/or converters may be required, this can lead to a more costly, unsafe, and unsightly, undesirable proliferation of power outlets and power cords. Further, in situations where the distance between devices is desired to exceed the maximum recommended lengths, a 110 or 220 volt power outlet may not even be available.
It would be desirable then if a bus regenerator, either a bus repeater but particularly a bus converter, could be provided for an SCSI or other information bus. Such a regenerator would desirably allow increased maximum bus lengths, preferably with conversion of the bus, without creating the need for additional power outlets and power cabling with their associated cost, clutter and safety concerns.